Project Summary Although acute pain and acute itch sensations serve essential protective functions, certain pathological conditions can lead to debilitating chronic pain and chronic itch disorders that greatly impair quality of life. A better understanding of the neural mechanisms underlying pain and itch is essential for developing better therapeutic options for these disorders. Despite recent advances in our knowledge of peripheral pain and itch differentiation, a few functional imaging studies in humans have shown significant overlap between brain regions associated with pain and itch activity, raising questions as to how these sensory modalities are distinguished in the central nervous system (CNS). At present, the circuitry and cellular mechanisms by which pain and itch are similarly or differently processed in the CNS remain unclear. My preliminary results show strong activation of the lateral habenula (LHb), a region important for integrating negative valence information and driving motivated behaviors, in response to trigeminal pain or itch stimuli. Given the distinct stereotyped behaviors produced by pain and itch sensation, the LHb may play an important role in differentiating pain and itch. Alternatively, the LHb may process shared aversive signals associated with both sensations. Nevertheless, the precise circuits in which the LHb functions to shape pain and itch sensory experiences have not been defined. In the proposed work, I will determine the divergent or convergent functional role of the LHb in trigeminal pain and itch pathways. In Aim 1, I will determine whether trigeminal pain- and itch-activated LHb populations are shared or divergent by comparing co-expression of known molecular markers in the LHb. To further establish the extent of overlap or segregation between these populations, I will combine genetic trapping of activated neurons and histological techniques to directly compare pain and itch activation in the same LHb. In Aim 2, I will use viral tracing to determine the presynaptic inputs and downstream projections of pain- and itch-activated LHb neurons, and gain insight into the differences and/or similarities between neural pathways mediating pain and itch sensations. Lastly, in Aim 3, I will determine the functional role of the LHb in mediating pain and itch sensations/behaviors. Specifically, I will investigate the dynamic activity of pain- and itch-activated LHb neurons by using in vivo calcium imaging. I will also test the requirement of these LHb populations for pain/itch sensation and behavior by selectively ablating them. The anticipated results of this proposal will improve our understanding about how pain and itch sensations are divergently or convergently processed in the CNS. Given the known role of the LHb, my work may reveal novel insight into the neural circuits mediating the affective-motivational dimensions of pain and itch.